The accountability of the subjects of international law for their acts and omissions, and the consequences thereof, is fundamental to the effectiveness and legitimacy of international law. Yet, the international environmental accountability of states remains a difficult and controversial topic, for both doctrinal and policy reasons. With few exceptions, states have not addressed this question in international environmental treaties, neither have international courts been burdened with cases or requests for advisory opinions on questions of environmental responsibility or liability of states. This article provides an analysis of the relationship between international environmental law and state responsibility, including a discussion of primary obligations, environmental harm, and the standard of care/due diligence. For the purpose of this article, accountability is used as an “umbrella term” and refers both to the responsibility of states and international organizations for internationally wrongful acts, as well as to their liability to make reparation for harm and damages resulting from their activities. Accountability for internationally illegal acts, such as breach of a treaty or the violation of customary law rules, is relatively well developed in general international law under the concept of state responsibility, though not in a codified, treaty-based manner and some uncertainties exist. In general, state responsibility refers to the accountability of a state for a violation of international law and is premised upon an internationally wrongful act which can be attributed to a state. Such internationally wrongful act can arise from the breach of an international legal obligation (“primary law”), established by treaty law (e.g. bilateral or multilateral environmental agreements) or by a customary norm of international law (for example the prohibition of environmental harm). The consequences of international responsibility for a wrongful act are the obligation of the wrong-doer to cease that act, to offer assurances of non-repetition, and to make full reparation of the injury caused by the internationally wrongful act, including compensation for environmental damage. On the other hand, rules for strict liability for environmental harm resulting from lawful activities are not so well established, and remain singular and exceptional. In general, states have shown little interest in setting up a general liability framework for environmental damage and only very few international environmental regimes impose liability and compensation schemes on states. Some issue-specific international environmental treaties impose civil liability on private actors, but do not establish a compensation scheme for states. Those liability rules are not used to prevent pollution or damage, but serve as a “backstop” to provide access to compensation when damage occurs despite the implementation of treaty rules that aim at preventing damage. Such damage can occur by accident or non-accidentally. In the later cases, compensation can be rewarded where a state did not exercise proper care so as not to cause adverse environmental effects to other states or areas beyond national jurisdiction or to prevent others in its territory from causing such effects. This standard is captured by the doctrine of “due diligence” which has been defined to require what a responsible government can be reasonably expected to do under normal conditions in a similar situation; applying its best practicable and available means. Due diligence does not refer only to unlawful omissions by a state, but also predominantly to the obligation of conduct to show proper care by exercising good government, management and control, for example, through enacting necessary legislation, effective law enforcement to prevent or terminate illegal activities.